Heme Synthesis : Synthesis of Porphyrin Molecule

Heme is a Porphyrin molecule. The porphyrins found in nature are compounds in which various side chains are substituted for the 8 hydrogen atoms numbered in Porphin nucleus. This notes explain about Heme synthesis in animals and plants.

Steps of Heme synthesis:

Both chlorophyll, the photosynthetic pigment of plants, and heme, the iron protoporphyrin of hemoglobin in animals are synthesized (Heme synthesis) in living cells by a common pathway.

Step 1: Formation of δ-amino Levulinic acid (ALA):

The precursor molecule for the heme synthesis is simplest and non-essential and optically inactive amino acid Glycine and the TCA cycle intermediate Succinyl~coA enzyme. Glycine condenses with Succinyl~coA. It forms δ-amino Levulinic acid. This reaction catalyzed by ALA synthatase. This reaction is takes place in mitochondria. This is the rate limiting step of this pathway.

Glycine + Succinyl~coA → δ-amino Levulinic acid + CO2 + CoenzymeA

Step 2: Formation of Porphyrinogen:

This reaction takes place in Cytosol. The dehydration of two molecules of ALA to form Porphobilinogen by the enzyme ALA dehydrase. The enzyme is inhibited by heavy metal ion lead.

2 (δ-amino Levulinic acid) → Porphobilinogen + H2O

Step 3: Formation of Uroporphyrinogen:

The condensation of four molecules of porphyrinogen. It gives Uroporphyrinogen-III. This reaction catalyzes by Uroporphyrinogen-I synthatase and uroporphyrinogen-III cosynthatase takes place in cytosol.

4 (Phorphobilinogen) →Uroporphyrinogen-III

Step 4: Formation of Heme:

UroPorphyrinogen-III is converted into Heme by a series of decarboxylation and oxidation. Finally the Uro-porphyrinogen-III converted into ProtoPorphyrinogen oxidase. The enzyme ProtoPorphyrin decarboxylase and protoporphyrin oxidase and protoporphyrinogen oxidase. The protoporphyrin-II is modified into heme by substituting the ferrous ion (Fe+3) by the enzyme Ferrochelation.